The present invention relates generally to an anti-skid brake control system for a wheeled vehicle for controlling braking pressure to be applied to a wheel cylinder or wheel cylinders in a hydraulic brake system. More particularly, the invention relates to a fail-safe system for an anti-skid control system which responds to abnormalities in the power supply to the anti-skid brake control system by performing a back-up operation.
In general, supply voltage for an anti-skid brake control system is regulated by a power regulator so as to be maintained at a constant voltage. The power regulator generally compensates for variations in the vehicle battery voltage and supplies a constant voltage to the electric or electronic circuit which, otherwise, might malfunction due to variations in the power supply voltage to the digital or analog ICs or the like.
Such power regulation circuitry has been disclosed in Published European Patent Application No. 0075931. In this disclosure, a fail-safe system in a power regulation circuit is provided for performing fail-safe operation even when the power regulation circuit is damaged and thereby the supply voltage from the regulation circuit varies significantly. The fail-safe system in the power regulation circuit includes a monitor power supply circuit independent of the power regulation circuit. The monitor power supply circuit is designed to produce a reference signal to be compared with the output of the power regulation circuit by a fail-safe circuit. The fail-safe circuit produces an error signal when the output signal value drops below a given threshold defined by the reference signal value to activate an alarm signal generator and disconnect an actuator controlled by the control system in order to prevent the actuator from malfunctioning due to malfunction of the control system.
In such power regulator circuitry, it is possible for the regulator circuit to detect a drop in the battery voltage below the given threshold during anti-skid brake control. For instance, relatively high power is required to operate an actuator which controls a pressure control valve for increasing, decreasing or holding constant the fluid pressure in a wheel cylinder. It may cause the battery voltage to drop below the given threshold briefly. Such power drop is only temporary, however and generally recovers shortly thereafter. However, in the aforementioned power regulator system in the prior art, fail-safe operation may be performed despite the anti-skid control operation state. Therefore, upon power drop due to actuation of the actuator during anti-skid brake control operation, the power source, i.e. vehicle battery, may be disconnected from the anti-skid brake control system, thus disabling the latter. Thus, after the fail-safe system becomes operative, anti-skid brake control is disabled, whereby the brake system applies fluid pressure to the wheel cylinders solely in accordance with depression of a brake pedal.
In such cases, the driver may apply an excessively high pressure to the brake pedal, relying on the anti-skid control system. Therefore, when the anti-skid control is disabled, the wheels may skid or lock, dangerously prolonging the braking distance.